I just finished reading a book series. The name of the book is Inside Threat by Jason Elam. In the book the main character is Christian, the girl that he loves is Muslim. His battle is because of their faith they can't marry each other. Why is that? Is it because within those religions they aren't allowed to marry outside of them? Any info would be helpful.

The Quran to my knowledge specifically talks about not forcing anyone into conversion. Of course it does happen in real life, because of people and their flaws, etc. To my knowledge there is no religous backing to blocking religious intermarriage, but there most definitely be could be a cultural one.

There are two kinds of marriage. civil which does not involve religion and then there is a religious marriage that involves the religion in the ceremony. If the two people are not of the same religion then a religious ceremony won't work.